Hot-melt inks are characterized by being solid at room temperature and molten at an elevated temperature at which the hot-melt ink is applied to a substrate. Hot-melt inks are widely used in ink jet printing, and have also been suggested for use in fiexographic, intaglio and gravure printing.
Ink jet printing is a well-known process for the non-contact printing of substrates such as paper, plastic films, metal foils and the like. In essence, ink jet printing ejects a stream of liquid ink through a very small orifice, and thereafter, at a certain distance from the orifice known as the breakup distance, the stream separates into minute uniformly-sized droplets. The ink droplets travel through the air until they hit a substrate, whereupon the ink forms an image on the substrate.
Various technologies have been developed to direct jet ink in an image-wise fashion from the printhead of a printing device to a substrate. In one technology, called drop-on-demand, the printhead passes over a substrate and ejects droplets of ink only when and where ink is desirably deposited on the substrate. Drop-on-demand technology is commonly employed in desktop ink jet printers.
In contrast, in a process known as continuous stream jet printing, the printhead is constantly ejecting ink droplets as it passes over a substrate, or as the substrate passes before the printhead. A guidance system is stationed between the printhead and the substrate, so ink droplets are directed either to a specific location on the substrate or to a recirculation gutter if the droplets being ejected should not be allowed to contact the substrate. A typical continuous stream ink jet printer employs inks that can be given an electric charge, and the guidance system is an electrostatic field that will interact with and direct the charged ink droplets to a desired location. Continuous stream jet ink printing is more commonly seen in industrial printing than in desk top printing.
Jet inks suitable for either drop-on-demand or continuous stream ink jet printing can be classified either as liquid jet inks or hot-melt jet inks. Either type of ink typically contains both colorant and carrier, where the carrier is a material that dissolves or suspends the colorant. A liquid jet ink is liquid at room temperature, and is typically at about room temperature while being stored in a printhead prior to being ejected. A simple liquid jet ink is composed of an aqueous carrier and a water-soluble dye as the colorant. After a liquid jet ink contacts a substrate, the solvent typically evaporates or wicks away from the colorant, leaving the colorant visible at the site where the ink initially contacted the substrate.
In contrast, a hot-melt jet ink is solid at room temperature, and is heated to a molten state prior to being ejected from an ink jet printhead. Upon contacting the substrate, which is typically at room temperature, the molten hot-melt ink will cool and solidify. A simple hot-melt ink is composed of wax as the carrier and a pigment or dye as the colorant. All, or nearly all, of the components of a hot-melt ink remain at the site where the molten ink contacts the substrate, i.e., there is little or no wicking or evaporation of a hot-melt ink.
An ink composition useful in jet ink printing should have certain properties. It is highly desirable that the ink display a consistent breakup length, droplet viscosity, and at least in continuous stream jet printing, a constant droplet charge under the conditions employed during the jet ink printing process. To meet these requirements, the jet ink composition must have stable viscosity, stable resistance properties, and should not dry out upon aging.
A major problem with liquid jet inks arises because they contain substantial amounts of water and/or organic solvent, which evaporate upon standing so that these inks dry out and cake. This can cause blocking of the printhead orifice(s). A further problem is that loss of volatile solvents causes the inks to increase in viscosity, which will cause substantial changes in the performance of the inks. Also, a porous substrate such as paper tends to cockle and/or distort when printed with high quantities of liquid jet ink. In addition, organic solvents in a liquid jet ink can evaporate after contacting the substrate, and this may cause health problems for some persons nearby.
Another problem associated with the presence of liquid solvents in a liquid jet ink is that these solvents cause the colorant to bleed into the printed substrate, which is typically porous, with the consequence that the printing displays poor resolution. While specially coated porous substrates may overcome this problem, such special substrates are expensive and not generally necessary for other types of printing, e.g., reprographic printing, which work fine with "plain paper", i.e., standard non-coated sheet. At least in an office setting, it is highly desirable that all printing, including ink jet printing, be done on "plain paper" or standard transparencies.
Hot-melt inks offer a number of advantages over liquid inks. For example, when liquid ink is used to deposit colorant on a porous substrate, the colorant tends to be carried into the substrate as the liquid carrier wicks into the substrate. This causes a reduction in print density and some loss in print resolution. In contrast, the rapid solidification of a hot-melt ink ensures that the colorant is fixed to the surface of the substrate, with a corresponding increase in print density and resolution. A further advantage is that there is little or no cockle associated with the printing of hot-melt inks, which is in distinct contrast to printing done with liquid inks. Still another advantage is that hot-melt inks are easier to transport without spillage than liquid inks.
For several reasons, the adhesion of colorant to a substrate may also be superior in hot-melt printing. For instance, because all of the carrier in a hot-melt ink stays with the colorant at the surface of the printed substrate, rather than evaporating or wicking away from the colorant as occurs in printing with liquid inks, a hot-melt carrier is more available to assist in fixing the colorant to the substrate surface. Also, carriers which are solid at room temperature will naturally have better fixing properties than liquid carriers.
Looking specifically at jet ink printing, hot-melt inks offer the advantage of having essentially no volatile components. Thus, there is no evaporation of components in a hot-melt ink, and so no corresponding problems with changes in ink viscosity, caking and health risks due to solvent evaporation, which are seen with liquid jet inks.
To a significant extent, the properties of the carrier determine the properties of a jet ink. The prior art discloses several materials that may be used as a carrier, sometimes called a vehicle, a binder or a solid organic solvent, in hot-melt jet inks. U.S. Pat. No. 3,653,932 discloses to use diesters of sebacic acid (a solid linear C.sub.10 dicarboxylic acid) and paraffinic alcohols having 12 or less carbons. U.S. Pat. No. 4,390,369 discloses to use natural wax. U.S. Pat. No. 4,659,383 discloses to use C.sub.20-24 acids or alcohols. U.S. Pat. No. 4,820,346 discloses to use aromatic sulfonamides. U.S. Pat. No. 4,830,671 discloses to use short-chain polyamides. U.S. Pat. No. 5,151,120 discloses to use the ethyl ester of stearic acid (a solid linear, C.sub.18 carboxylic acid). U.S. Pat. No. 5,354,368 discloses to use tall oil rosin. The foregoing are exemplary of the prior art directed to hot-melt ink carriers.
Despite the significant amount of research that has been done in the area of carriers for hot-melt inks, there remains a need in the art for superior carrier materials useful in hot-melt inks, and for inks having such carrier materials.